1. Field of the Invention
The present invention is related to a current mode boost converter, and more particularly, to a current mode boost converter that adaptively adjusts an activation condition of a pulse frequency modulation mode according to a mean value of an inductor current or AC components of a slope compensation current and the inductor current.
2. Description of the Prior Art
An electronic device generally includes various components requiring different operating voltages. Therefore, a DC-DC voltage converter is essential for the electronic device to adjust (step up or step down) and stabilize voltage levels. Based upon different power requirements, various types of DC-DC voltage converter, originating from a buck (step down) converter and a boost (step up) converter, are developed. Accordingly, the buck converter can decrease an input DC voltage to a default voltage level, and the boost converter can increase an input DC voltage. With advances in circuit technology, both the buck and boost converters are varied and modified to conform to different system architectures and requirements.
For example, please refer to FIG. 1, which is a schematic diagram of a boost converter 10 of the prior art. The boost converter 10 includes an input end 100, an inductor 102, a switch transistor 110, an output module 120, an output end 130, a feedback module 140, an error amplifier 142, a voltage reduction circuit 144, a pulse width modulation (PWM) compensation circuit 146, a current sensor 150, a current sense circuit 152, a slope compensation circuit 160, a first comparator 170, a second comparator 180, a third comparator 190, an oscillator 192 and a modulation control circuit 194. The input end 100 is utilized for receiving an input voltage VIN. The switch transistor 110 is utilized for determining whether an inductor current IL of the inductor 102 charges the output module 120 according to a second switch signal SW2. The output module 120 includes a diode 122, an output resistor 124 and an output capacitor 126, and is utilized for generating an output voltage VOUT based on the inductor current IL, a conduction state of the switch transistor 110 and frequency responses of the diode 122, the output resistor 124 and the output capacitor 126. The feedback module 140 is utilized for generating a division voltage of the output voltage VOUT as a feedback signal VFB. The error amplifier 142 is utilized for amplifying a difference between the feedback signal VFB and a first reference voltage VREF1 to generate a difference voltage ΔV. The voltage reduction circuit 144 is utilized for generating a division voltage VREF1′ at a level slightly lower than the first reference voltage VREF1. The second comparator 180 is utilized for comparing the division voltage VREF1′ and the feedback signal VFB to generate a PWM trigger signal TR_PWM. Other than the feedback scheme, the current sensor 150 generates a sensing current ISEN proportional to the inductor current IL. The current sense circuit 152 amplifies the sensing current ISEN to reconstruct the inductor current IL as a mirror inductor current IL_C. The slope compensation circuit 160 is utilized for generating a slope compensation current ISC. A summation of the mirror inductor current IL_C and the slope compensation current ISC is converted into a compensation voltage VC through a resistor R. The PWM compensation circuit 146 is utilized for compensating a frequency response of the boost converter 10 based on the difference voltage ΔV to generate a compensation result EAO. The first comparator 170 is utilized for comparing the compensation voltage VC and the compensation result EAO to generate a PWM signal VPWM. The second comparator 180 is utilized for comparing the feedback signal VFB and a division voltage VREF1 at a level slightly lower than the first reference voltage VREF1 to generate a PWM trigger signal TR_PWM. The third comparator 190 is utilized for comparing the compensation result EAO and a constant threshold voltage VTH to generate a pulse frequency modulation (PFM) trigger signal TR_PFM. The oscillator 192 is utilized for generating an oscillating signal VOSC. Finally, the modulation control circuit 194 determines an operation mode of the boost converter 10 based on the PWM trigger signal TR_PWM, the PFM trigger signal TR_PFM, the PWM signal VPWM and the oscillating signal VOSC, and accordingly generates a first switch signal SW1 sent to an amplifier 196. The amplifier 196 amplifies the first switch signal SW1 to generate a second switch signal SW2 sent to the switch transistor 110.
In short, the boost converter 10 determines whether to operate in a PWM mode or a PFM mode based on the inductor current IL. When the inductor current IL is relatively low, the boost converter 10 switches from the PWM mode to the PFM mode to reduce a switching loss of the boost converter 10 by minimizing switching operations of the switch transistor 110. The boost converter 10 generates the PWM trigger signal TR_PWM and the PFM trigger signal TR_PFM according to the sensing current ISEN and the feedback signal VFB, and accordingly determines whether to operate in the PWM mode or the PFM mode.
For example, under a condition that the input voltage VIN and the output voltage VOUT are invariant, the larger the inductor 102, the higher a current threshold Ith1 specifying a decision boundary from the PWM mode to the PFM mode, as illustrated in FIG. 2. In the worst case, the current threshold Ith1 is even higher than a current threshold Ith2 specifying a decision boundary from the PFM mode to the PWM mode, causing the boost converter 10 to oscillate between the PWM mode and the PFM mode and malfunction. To prevent the mode oscillation, one approach is to decrease the threshold voltage VTH. However, the lower threshold voltage results in a very small current threshold Ith1 when inductance of the inductor 102 is relatively small, implying that the PFM mode is inaccessible.
Therefore, fixing the decision boundaries between the PWM mode and the PFM mode has been a major focus of the industry.